The formation of "breathable" films that are gas/vapor permeable and liquid impermeable by stretching a precursor film containing a high level of filler material is known, and described, for example in U.S. Pat. No. 4,472,328, assigned to Mitsubishi Chemical Industries, Ltd. The breathable polyolefin film of the Mitsubishi patent is prepared from a polyolefin/filler composition having from 20 percent to 80 percent by weight of a filler material, such as calcium carbonate. A liquid or waxy hydrocarbon polymer elastomer such as a hydroxy-terminated liquid polybutadiene was described as producing a precursor film that could be monoaxially or bi-axially stretched to render the film breathable. The breathable film of the Mitsubishi patent is further mentioned in Great Britain Patent No. 2,115,702, assigned to Kao Corporation. The Kao patent describes the use of the breathable film of Mitsubishi as a diaper backsheet that prevents the leakage of liquids.
U.S. Pat. No. 4,350,655, assigned to Biax Fiber Film, ("the Biax '655 patent") describes a porous polyolefin film containing at least 50 percent by weight of a coated inorganic filler. The precursor film is formed without the addition of an elastomer by employing an inorganic filler that is surface coated with a fatty acid ester of silicon or titanium. The precursor film is then stretched between horizontally grooved rollers. Cold stretching of the precursor film at a temperature below 70.degree. C. produces a porous film. The resulting films are generally both vapor and liquid permeable. However, Example 3 of the Biax patent describes a film that is vapor permeable and liquid impermeable.
A problem with polymer films rendered breathable by incorporation of filler materials is that the use of fillers also reduces the strength of the films. This reduction in tensile strength is disadvantageous in situations where the film must have sufficient strength to prevent tearing, for example to allow for attachment of tapes or the incorporation of fastening systems or elastic leg bands.
U.S. Pat. No. 3,840,418 to Sabee ("the Sabee '418 patent") describes a method for forming a sanitary product provided with sections having increased pin- or tape-retention strength in which a single synthetic resin is extruded by a selective extrusion process that selectively thickens the film web of resin at localized points where strength is needed. The Sabee patent does not discuss the use of filled polymer compositions and does not adjust the thickness of the various portions of the sanitary product to control vapor permeability.
One proposed manner of increasing the physical properties of filled and stretched polymer sheets is described in U.S. Pat. No. 4,777,073, assigned to Exxon Chemical Patents, Inc. ("the Exxon '073 patent"). The Exxon '073 patent describes the melt embossing of polymer/filler precursor films, such that portions of the film will have reduced thickness. When the films are activated by stretching, the portions of reduced thickness exhibit higher permeability and the portions with non reduced thickness exhibit higher tear strengths.
Although the films described in the Exxon '073 patent provide high permeability and necessary strength for the attachment of fasteners and the like, they do so at relative high thickness. High thickness increases the cost of the films and reduces the films' flexibility and soft feel. Films described in the Exxon '073 patent are also still deficient in strength in certain applications in consumer disposables, where the maker(s) of these disposables subject these films to secondary deformation steps.
With all the technical efforts focused on developing better film, there still remains a need for superior polymer film products. In view of this need, applicants have developed polymer films of substantially constant thickness, providing areas of improved vapor permeability and contiguous areas exhibiting increased tensile and tear strengths, which polymer films can be economically and simply formed in a single co-extrusion step.